My contract reads "....not responsible for damage to any hidden conditions, including, but not limited to, underground gas, water, telephone, electric, cable t.v. or other utilities, drains, pipes, wires, septic systems, sprinkler or other irrigation systems. The property owner agrees to inform.......of any hiddne conditions in the tree(s), including, but not limited to, unnatural objects, such as steel posts, cables, metal hooks or attachments. Should the property owner not inform......., the property owner agrees to be liable for any personal or property damage suffered as a result of such hidden or unnatural conditions."
I run the Sandvik wheels on both of my grinder. I don't damage teeth with concrete, bricks, rocks, etc. it's metal that breaks them. I charge $12 per tooth that is damaged. In the last year I have only broken 20 teeth in over $275 hours of grinding.
I have an $80 minimum to go an grind any stump. I charge $3 per inch from 1-30 inches, $4 per inch 31-40 inches, $5 per inch 41-50 inches, $6 per inch 51-60 inches and $7 per inch 61 and over. If you do the math (pi x R squared) you'll find that a 60 inch stump has 4 times the surface area of a 30 inch stump.......and I charge appropriately.
I charge $250 per hour with my large grinder and $150 per hour with my small grinder. I have a $450 minimum for hourly work with the large grinder and a $300 minimum for hourly work with the small grinder.
I've had customers not like the idea of paying for teeth but they have all paid so far. Most under stand that you can't tell if there is a hidden object and many don't want you to lose money.
My $.02